The invention relates to a system comprising at least one encoder for coding a digital signal and at least one decoder for decoding a coded digital signal, which encoder is provided with
an input for receiving the digital signal,
first data processing means, coupled to the input, for generating a first coded digital signal,
first encoder feedback means for feeding back at least a section of the first data processing means, which first encoder feedback means comprise encoder switching means, of which a main contact is coupled to the input and of which a first switch contact is coupled to the first data processing means via first encoder memory means,
reduction means, coupled to the input, for reducing the received digital signal,
second data processing means, coupled to the reduction means, for generating a second coded digital signal, and
second encoder feedback means for feeding back at least a section of the second data processing means, which second encoder feedback means comprise second encoder memory means and are coupled to a second switch contact of the encoder switching means via encoder prediction means, and which decoder is provided with
first data reprocessing means for processing a first coded digital signal,
first decoder memory means which are coupled to a first switch contact of decoder switching means, of which a main contact is coupled to the first data reprocessing means,
second data reprocessing means for processing a second coded digital signal, and
second decoder memory means which are coupled to the second data reprocessing means and, via decoder prediction means, to a second switch contact of the decoder switching means.
Such a system is disclosed in "Coded representation of picture and audio information", "TM1 Compatibility Experiments" by I. Parke, ISO-IEC/JTC1/SC29/WG11, MPEG92/291, in particular FIG. 1 (the encoder) and FIG. 2 (the decoder) thereof. The encoder, which is based on layered coding, comprises the input for receiving the digital signal to be coded, such as, for example, a television signal made up of picture elements (pixels or pels), and the first data processing means, coupled to the input, for generating the first coded digital signal. Furthermore, the encoder comprises the first encoder feedback means for feeding back at least a section of the first data processing means in order to make the coding proceed more efficiently. The first encoder feedback means comprise the encoder switching means, of which a main contact is coupled to the input and of which a first switch contact is coupled to the first data processing means via the first encoder memory means. The encoder also comprises the reduction means, coupled to the input, for reducing the received digital signal, the second data processing means, coupled to the reduction means, for generating a second coded digital signal, and the second encoder feedback means for feeding back at least a section of the second data processing means in order to make the coding proceed more efficiently. The second encoder feedback means comprise the second encoder memory means and are coupled to a second switch contact of the encoder switching means via the encoder prediction means.
Such a system, provided with first and second data processing means, is, as it were, made up of two layers: the first data processing means generate the first coded digital signal, which has the highest resolution because said signal is obtained by processing the received digital signal, and the second data processing means generate the second coded digital signal, which has the lowest resolution because said signal is obtained by processing the digital signal received by the reduction means. Both signals are then passed, by means of multiplexing and demultiplexing to the decoder, which is based on layered decoding and which, in the case of decoding based on higher resolution, uses both signals and which, in the case of decoding based on low resolution, uses only the second coded digital signal. The decoder comprises the first data reprocessing means for processing a first coded digital signal, the first decoder memory means, which are coupled to a first switch contact of the decoder switching means, of which a main contact is coupled to the first data reprocessing means, the second data reprocessing means for processing a second coded digital signal, and the second decoder memory means, which are coupled to the second data reprocessing means and, via decoder prediction means, to a second switch contact of the decoder switching means.
In this arrangement, the encoder switching means and the decoder switching means are designed as switch-over means, that is to say the main contact is connected through either to the first switch contact or to the second switch contact. If, in subordination to a "system controller", the main contact of the encoder switching means is connected through to the first switch contact, the preceding high-resolution picture stored in the first encoder memory means is used in order to predict the new high-resolution picture. If, in subordination to the "system controller", the main contact of the encoder switching means is connected through to the second switch contact, the instantaneous low-resolution picture is used in order to predict the new high-resolution picture. The information needed for this purpose and originating from the "system controller" is also passed to the decoder by means of multiplexing and demultiplexing so that the decoder switching means are in a synchronous state with the encoder switching means.
Such a known system has the disadvantage that it codes with inadequate efficiency.